NMR will be used to probe the origin of the spectroscopic differences between reduced vertebrate and chloroplast [2Fe-25] ferredoxin and the conformational change that reduction induces to the protein in vertebrate ferredoxins. Selective isotopic labeling will be used to assign the paramagnetically shifted resonances, which are a sensitive indicator of cluster geometry. Density functional calculations on simple models of the reduced [2Fe-2S] cluster will allow analysis of the nature of geometric differences between chloroplast and human ferredoxin by comparison with existing spectroscopic data. More sophisticated models can then be developed, which will be used to calculate paramagnetic shifts for comparison with experiment, thereby generating a model which is consistent with all the data. Global 13C and 15N labeling and standard 2- and 3-D NMR techniques, as well as methods which exploit the paramagnetism, will be used to obtain solution structural information of the diamagnetic region of the protein in both redox states, providing insight into the reduction-induced conformational change. This will be melded onto the active site model developed above, thus giving a complete picture of the reduced ferredoxin structure. Given the absence structural information on any reduced ferredoxin, and that the reduced and oxidized structures are clearly different, this is the only valid approach to solution structure determination. Mutagenesis studies will be used to probe the factors which dictate the reduced [2Fe-2S] cluster geometry. This work will provide needed insight into how reduction alters ferredoxin's affinity for its redox partners in the steroid hormone biosynthetic pathway.